Recently, in a radio communication system such as a mobile phone system (cellular system), the next generation radio communication technology has been discussed in order to achieve a further high speed and large capacity in radio communication. For example, in 3rd Generation Partnership Project (3GPP) which is a standardization organization, a communication standard referred to as Long Term Evolution (LTE) or a communication standard referred to as LTE-Advanced (LTE-A) based on the LTE radio communication technology is suggested.
The latest communication standard completed in 3GPP is Release 10 corresponding to the LTE-A, and has functions obtained largely by extending functions of Release 8 and Release 9 corresponding to the LTE. A discussion about the main parts of Release 11 obtained by further extending Release 10 has been presently ended, and detailed parts for completion have been finished. Additionally, a discussion about Release 12 has been started. “LTE” is added to the LTE and the LTE-A, and includes other radio communication systems obtained by extending the LTE and the LTE-A, as long as there is no particular separation.
Release 12 of 3GPP includes various technologies, and there is a small cell as one of these technologies. The small cell is a relatively small cell and a concept competing with a macro cell which is relatively large cell. The macro cell is formed by a relatively large radio base station, but the small cell is formed by a relatively small radio base station. Here, a “cell” is a term indicating a range which is covered by a radio base station in order to cause a radio terminal to transmit and receive a radio signal. However, the radio base station and the cell are concepts which almost correspond to each other, and thus a “cell” and a “radio base station” can be appropriately changed and read in the description of this application.
Effects obtained by introducing a small cell are considered. For example, a small cell is disposed at a location such as a hot spot, which has heavy communication traffic, and thus it is possible to reduce a load of a macro cell. If the radio terminal transmits a signal to a small cell which is nearer than a macro cell far away, it is possible to suppress an increase of transmission power and an effect in that good communication characteristics are obtained can be also expected. The small cell is considered as a component technology which can solve various problems included in the current or the future radio communication system, and will be continuously discussed as a promising technology in 3GPP.
In 3GPP, as one of the technologies associated with the small cell, a study of dual connectivity has been started. In the dual connectivity, a radio terminal is connected to a plurality of radio base stations and simultaneously communicates with the plurality of radio base stations, and thus the radio terminal transmits or receives different information to or from the radio base stations at the same time.
FIG. 1 illustrates a conceptual diagram of the dual connectivity. As illustrated in FIG. 1, as one example of the dual connectivity, a case where a radio terminal (user equipment: UE) is connected to both of the macro cell and the small cell, and the like are considered when a plurality of small cells is disposed in a macro cell. For example, the dual connectivity enables the radio terminal to perform transmission and reception of different information with both of the macro cell and the small cell. Thus, it is possible to realize high-speed communication. A discussion regarding the dual connectivity has been just started in 3GPP. However, the dual connectivity allows the high speed and large capacity desired for future radio communication systems to be obtained, and thus it is expected that many discussions will be continuously repeated.
The dual connectivity is described in this application. However, a similar discussion can be performed in multiple connectivity of triple sources or more. Accordingly, the dual connectivity in this application may be recognized as a concept in which the multiple connectivity is included, and it is noted that the dual connectivity may be replaced with the multiple connectivity in this application.